This invention relates to a current to voltage amplifier circuit and, more particularly, to such a circuit used in a ground fault detection circuit.
Ground fault detection circuits are well known which will develop a detection signal that varies with very small currents being monitored. The current is amplified to produce a detection signal. When the detection signal exceeds a preselected level, a relay or the like is deenergized to disconnect power from the load. Such circuits are often used as ground fault, or short circuit, detectors.
Examples of low current detection circuits used in fault detectors and otherwise are shown in U.S. Pat. Nos. 3,815,013 of Milkovic; 3,851,216 of Clarke et al., 3,836,821 of Wahlgren et al.; 3,611,035 of Douglas; 4,263,637 of Draper et al. and 4,309,681 of Draper et al. Reference may be made to these patents for background and for more detailed information concerning the environment in which ground fault detection circuits are used and the nature of the problems encountered.
Briefly, one of the problems encountered in fault detectors is false fault detection due to ambient or line noise.
Another problem has been uncontrollable variations of trip levels due to temperature fluctuation and DC stability. Fluctuation in temperature has caused the inductance of the inductors in known circuits to decrease to levels at which the circuits response is affected. Negative coefficient thermistors and the like have been used to compensate for temperature variation with some success. However, the basic problem remains.
Ambient and line noise creates a particularily difficult problem due to the low magnitude of the currents being sensed. Known detectors lack the proper frequency response characteristics to minimize noise problems. In addition, known fault detection circuits use DC amplifiers which are susceptible to DC noise and DC supply voltage stability problems.